PAT2 Diagnosis of a large cell neuroendocrine carcinoma (NEC) by pleural fluid cytology, with subsequent analysis confirming metastasis from unexpected breast primary. (Pathology).

PAT2 DIAGNOSIS OF A LARGE CELL NEUROENDOCRINE CARCINOMA (NEC) BY PLEURAL FLUID CYTOLOGY, WITH SUBSEQUENT ANALYSIS CONFIRMING METASTASIS FROM UNEXPECTED BREAST PRIMARY William P. Daley, MD, E. Rhyne Flowers, MD, and Mithra Baliga, MD, Department of Pathology, University of Mississippi, Jackson.

Breast carcinomas with neuroendocrine differentiation are considered unusual variants representing 3.3% - 5.0% of all cases when defined by argyrophilia. Since the classical description of carcinoid of the breast by Cubilla and Woodluff, controversy has arisen concerning the criteria for the diagnosis of neuroendocrine carcinoma (NEC). Papolti et al described 7 major histologic patterns of NEC of the breast, among which mucinous carcinomas and so called mammary carcinoid constituted the two most common groups. The rare published cytologic reports describe mammary carcinomas with features of carcinoid tumor and poorly differentiated NEC comprised of medium to large cells diagnosed by fine needle aspiration biopsy. We present an unusual case of a pure NEC of large cell type with malignant cytologic features metastatic to the pleura that was diagnosed by pleural fluid cytology and confirmed by immunocytochemistry, hormone receptor studies, and histopathologic examination. A 61-year-old black female had undergone a left modified radical mastectomy on 5/98 for a 6.5 cm breast mass involving the upper outer quadrant. The histopathologic diagnosis of a moderately differentiated infiltrating carcinoma (Elston's grade 2/3) was rendered. One of thirty-two lymph nodes was positive for metastatic carcinoma. The patient subsequently developed pleural metastasis on 10/2,000. Pleural fluid cytology and a pleural biopsy revealed large cell NEC confirmed by immunochemistry. Following this diagnosis, a careful analysis of excised breast tumor from the mastectomy specimen showed similar cytomorphologic, histochemical and hormone receptor results confirming pleural metastasis from a pure large cell NEC primary in the breast. Pleural fluid cytology can be an extremely helpful tool in establishing the diagnosis of unsuspected metastatic neuroendocrine carcinoma of breast. Awareness of primary NEC of breast is essential during cytologic evaluation of pleural fluid in order to avoid unnecessary extensive clinical work up to exclude othe r primary sites.